This invention relates to a system for measuring the natural gamma radiation of surface and subsurface formations.
Various methods and apparatus have been utilized in the well logging art to study the radioactive properties of subsurface formations, both where the radioactivity is natural and where it is artifically induced. Logs of such properties aid in the study of the nature of the subsurface formations, particularly in exploration for minerals and hydrocarbon deposits. Certain elements in the subsurface formations exhibit distinctive properties which are measurable in situ. Of the many elements that occur, potassium (K), uranium (U), and thorium (Th) are important natural sources of gamma radiation. Each of these elements either contains or radioactively decays to, radioactive isotopes which emit gamma radiation at characteristic energy levels. The neutral gamma-ray spectrum of a given formation therefore exhibits peaks of intensity at energies corresponding to the potassium, uranium, and thorium content of the formation.
In U.S. Pat. No. 3,940,610 to Dennis et al, a borehole logging system employs a gamma-ray detector for measuring the total natural gamma radiation within the borehole. Three energy band selectors discriminate this measurement of the gamma-ray detector into potassium, uranium, and thrium energy band signals centered about the energy levels at which potassium, uranium, and thorium exhibit peak gamma radiation intensities.
These potassium, uranium, and thorium energy band signals are then applied to a stripping unit having potassium and uranium channels. The potassium channel strips the potassium gamma radiation measurement, as represented by the potassium energy band signal, of the influence from the gamma radiation from uranium and thorium, Further, the uranium channel strips the uranium gamma radiation measurement, as represented by the uranium energy band signal, of the influence from the gamma radiation from thorium.
For a further discussion on the applications of field gamma-ray spectrometry as a geological mapping and exploration tool reference may be had to an article entitled "Techniques of field gamma-ray spectrometry" in the Mineralogical Magazine, December 1981, Vol. 44.